Interleukin 6 [F. Takatsuki et al., Cancer Research, 50, 2885-2890 (1990)], leukemia inhibitory factor [D. Metcalf et al. Blood, 76, 50-56 (1990)], stem cell factor [P. Hunt et al. Blood, 80, 904-911 (1992)], macrophage colony stimulating factor (M-CSF; Japanese Published Examines Patent Application No. 11705/94) and thrombopoietin [de Sauvage et al. Nature, 369, 533 (1994)] are known as substances which possibly promote platelet production. Furthermore, conagenin [Japanese Cancer Association #2235 (1992)], Y25510 [The 113rd annual meeting of Pharmaceutical Society of Japan, PB13-22 (1993)], 2-pyranone derivatives [Japanese Published Unexamined Patent Application No. 213758/93], and FK565 (WO93/23066) are known as low molecular weight substances which possibly promote platelet production.
It is known that hG-CSF is one of the polypeptides essential for hemopoietic stem cell growth and differentiation leading to the formation of various types of hemocytes, and exerts growth-promoting effect on most granulocytes and in particular neutrophils.
As a modified polypeptide exhibiting hG-CSF activity wherein groups are chemically modified with a chemical modifying agent, a chemically modified hG-CSF obtained by modifying at least one amino group of the polypeptide exhibiting hG-CSF activity with a polyethylene glycol derivative is known (Japanese Published Unexamined Patent Application No. 316400/89, WO90/06952, Japanese Published Unexamined Patent Application No. 32559/92). It has not been known that these chemically modified hG-CSF polypeptides exert a platelet production promoting effect.